Prefix substitution support for cpp(1)

Hi all,
I have a patch in one of my working trees to allow changing the prefix
for __FILE__ based on prefix patterns. This allows building packages
without leaking the full path, so that changes src and obj location
don't alter build.

The primary reason to develop this was to be able to check for
.buildlink references in pkgsrc, which has very similar problems.
It also helps if diagnostic messages actually refer to the files in
/usr/pkg/include.

Would this be considered generally useful? In that case I will clean up
the patch and prepare some documentation for it.

Joerg